1. Field of the Invention
The present invention relates generally to heat exchangers and, more particularly to modularization for stacked plate heat exchangers.
2. Description of the Related Art
The feedwater for steam generators in nuclear power plants is typically preheated before being introduced into the secondary side of the steam generators. Similarly, feedwater is preheated before being introduced into boilers for non-nuclear power plant applications. Feedwater heat exchangers are typically used for this purpose. Conventionally, heat exchanger designs are divided into two general classes; heat exchangers with a plate structure and those with a tube and shell structure. The major difference in the two classes, with regard to both construction and heat transfer, is that the heat transfer surfaces are mainly plates in one structure and tubes in the other.
The tube and shell heat exchanger in a number of feedwater heater applications employs a horizontal tubular shell having hemispherical ends. The inside of the horizontal shell is divided into sections by a tube sheet which is normal to the axis of the shell. More specifically, at one end of the shell, a water chamber section is defined that includes a water inlet chamber having a water inlet opening and a water outlet chamber having a water outlet opening. A plurality of heat transfer tubes are bent at their mid portions in a U shape and extend along the axis of the shell. These tubes are fixed to the tube sheet at both ends such that one end of each of the tubes opens in the water inlet chamber, while the other end opens in the water outlet chamber. The heat transfer tubes are supported by a plurality of tube supporting plates, spaced at a suitable pitch in the longitudinal direction of the tubes. An inlet opening for steam and a drain inlet and outlet are formed in the shell in the portion in which the tubes extend.
In operation, the feedwater coming into the feedwater heater from the water inlet chamber flows through the U-shaped heat transfer tubes and absorbs the heat from the heating steam coming into the feedwater heater from the steam inlet opening to condense the steam. The condensate is collected at the bottom of the shell and discharged to the outside through a drain in the bottom of the shell. Thanks to the cylindrical shape of the shell and the heat exchange tubes, the structure is well suited as a pressure vessel, and thus tube and shell heat exchangers have been used in extremely high pressure applications.
The most significant drawback of the tube and shell heat exchangers is their heavy weight when compared to the surface area of the heat transfer surfaces. Due to that, the tube and shell heat exchangers are usually large in size. Also, it is difficult to design and manufacture tube and shell heat exchangers when the heat transfer, flow characteristics and expense are taken into account.
A typical plate heat exchanger is composed of rectangular, ribbed or grooved plates, which are pressed against each other by means of end plates, which, in turn, are tightened to the ends of the plate stack by means of tension rods or tension screws. The clearances between the plates are closed and sealed with banded seals on their outer circumference and the seals are also used at the flow channels. Since the bearing capacity of the sleek plates is poor, they are strengthened with the grooves which are usually arranged crosswise in adjacent plates, wherein they also improve the pressure endurance of the structure when the ridges of the grooves are supported by each other. However, a more important aspect is the significance of the grooves for heat transfer; the shape of the grooves and their angle with respect to the flow, affect the heat transfer and pressure losses. In a conventional plate heat exchanger, a heat supplying medium flows in every other clearance between the plates, and a heat receiving medium in the remaining clearances. The flow is conducted in between the plates via holes located in the vicinity of the corners of the plates. Each clearance between the plates always contains two holes with closed rims and two other holes functioning as inlet and outlet channels for the clearance between the plates. The plate heat exchangers are usually constructed of relatively thin plates, when a small and light structure is desired. Because the plates can be profiled into any desired shape, it is possible to make the heat transfer properties suitable for almost any type of application. The greatest weakness in conventional plate heat exchangers is the seals which limit the pressure and temperature endurance of the heat exchangers. In several cases, the seals have impaired the possibility of use with heat supplying or heat receiving corrosive medium.
Attempts have been made to improve the plate heat exchanger construction by leaving out all of the seals and replacing them with soldered joints or welded seams. Plate heat exchangers fabricated by soldering or welding usually resemble those equipped with seals. The most significant external difference is the absence of tension screws between the ends. However, the soldered or welded structure makes it difficult if not impossible to nondestructively dissemble such heat exchangers for cleaning.
Attempts have been made to combine the advantages of the tube and shell heat exchanger and the plate heat exchanger in heat exchangers whose construction partly resembles both of these basic types. One such solution is disclosed in the U.S. Pat. No. 5,088,552, in which circular or polygonal plates are stacked on top of each other to form a stack of plates which is supported by means of end plates. The plate stack is encircled by a shell, the sides of which are provided with inlet and outlet channels for corresponding flows of heat supplying and heat receiving medium. Differing from the conventional plate heat exchanger, all flows into the clearances between the plates are directed from outside the plates. When the heat exchanger according to the publication is closed by welding, it is possible to attain the same pressures as when using a tube and shell heat exchanger, with the heat transfer properties of a plate heat exchanger.
International Publication WO 91/09262 purports to present an improvement on the foregoing publication, which more distinctly exhibits features typical of both plate heat exchangers and tube and shell heat exchangers. The circular plates are drawn together in pairs by welding them together by the rims of holes which form an inlet and outlet channel. By welding the plate pairs fabricated in the above manner together by the outer perimeters of the plates, a closed circuit is attained for the flow of one heat transfer medium. Differing from the conventional plate heat exchanger, this structure is welded and there are only two holes in the plates. The flow of another heat transfer medium is directed to every other clearance between the plates by means of a shell surrounding the stack of plates. In order to prevent the flow from running between the plate stack and the shell, seals are utilized which are primarily used as deflectors for the flow. Obviously, pressure endurance is not required of the deflectors. Due to the structure of the plate stack, it is difficult to implement the seals. Elastic rubber gaskets are suggested for the seals so that it is possible to dissemble the heat exchanger, e.g., for cleaning purposes.
The shell and tube heat exchanger currently used in nuclear power plants has a common design flaw that when tube degradation occurs, in an effort to minimize leakage, the only option is to plug the damaged tube resulting in a loss of thermal duty. The loss of thermal duty in the feedwater system is costly for nuclear power plants and eventually requires the replacement of the shell and tube feedwater heater. Another limitation of the shell and tube design is that the shell side inspection is typically limited to small hand holes and inspection ports and as a result corrosion/erosion damage is difficult to detect. Significant corrosion/erosion has been sustained by the internal baffling which can lead to (1) flow bypass and thermal performance degradation, and (2) tube wear due to flow induced vibration. Significant corrosion/erosion has also been observed on the inner shell surface of the shell and tube feedwater heater design.
Therefore, a new feedwater heater design is desired for long term, sustainable thermal duty and for improved long term component integrity relative to the current shell and tube feedwater heater design. Preferably, long term, sustainable thermal duty will be achieved by replacement or repair of the heat transfer surfaces, as needed, instead of requiring that the heat transfer surface be removed from service. Additionally, it is desirable to be able to increase the heat transfer capability of the feedwater heater to accommodate power plant uprates without replacing the entire feedwater heater.